The Cosmic Background Imager is located on a Chilean plateau at 16,700 feet.

The universe, up close and personal

Using a special instrument high in the Chilean Andes, Caltech cosmologists have uncovered the finest detail so far in the cosmic microwave background radiation (CMB), which originates from just 300,000 years after the Big Bang. The new images, in essence, are photographs of the cosmos before stars and galaxies existed, and reveal for the first time the “seeds” from which clusters of galaxies grew.

The observations were made with the Cosmic Background Imager (CBI), designed especially to make detailed, high-precision pictures to measure the geometry of space-time and other fundamental cosmological quantities.

The CMB provides a crucial experimental laboratory for understanding the universe’s origin and eventual fate, because in that remote epoch, matter had not yet formed galaxies and stars. Tiny density fluctuations at that time grew to produce all the structures in the universe today, including galaxy clusters, galaxies, stars, and planets. These density fluctuations gave rise to temperature fluctuations seen in the microwave background.

First detected in 1965, the CMB arose when matter had cooled enough for electrons and protons to combine to form atoms, at which point the universe became transparent. Before that time, the universe was an opaque fog because light couldn’t travel very far before hitting an electron.

The photons seen today with instruments like the CBI, the earlier COBE satellite, the BOOMERANG and MAXIMA experiments, and the DASI instrument (a sister project to the CBI that also makes high-precision interferometry measurements), have been traveling through the universe since first emitted from matter about 14 billion years ago.

Temperature differences observed in the CMB are so slight (about one part in 100,000) that it’s taken 37 years to get such finely detailed images as those from the CBI. Though first detected in 1965, the microwave background appeared to be smooth due to the limitations of the instruments available. The COBE satellite first demonstrated the CMB’s slight variations in the early 1990s.

The CBI results provide independent confirmation that the universe is “flat,” and also yield a good measurement of the amount of nonbaryonic “dark matter”—which differs from the stuff everyday objects are made of—in the universe. In addition, the results confirm the importance of “dark energy” in the universe’s evolution.

According to Anthony Readhead, Caltech’s Rawn Professor of Astronomy and principal investigator for the CBI, “These unique high-resolution observations give a powerful confirmation of the standard cosmological model. Moreover, this is the first direct detection of the seeds of clusters of galaxies in the early universe.”

The flat universe and the existence of dark energy add empirical credence to the “inflation” theory, which states that the universe grew from a tiny subatomic region during a period of violent expansion a split second after the Big Bang.

Because it sees finer detail in the CMB sky, the CBI verifies and goes beyond the recent successes of the BOOMERANG and MAXIMA balloon-borne experiments and the University of Chicago’s DASI experiment at the South Pole.

The BOOMERANG experiment, led by Andrew Lange, Caltech’s Goldberger Professor of Physics, demonstrated the universe’s flatness two years ago. Those findings, along with data from MAXIMA and DASI, also bolstered the inflation theory with accurate measurements of many cosmological parameters.

The fact that the CBI observations when compared with others are at very different resolution, and that the various observations are made with widely differing techniques, at different frequencies, and covering different parts of the sky, and yet agree so well, gives the findings great confidence.

The CBI is a microwave telescope array comprising 13 separate antennas, each about three feet in diameter, set up to act together as an interferometer. Located at Llano de Chajnantor, a Chilean plateau at 16,700 feet, it is the most sophisticated scientific instrument ever used at such an altitude—so high that the researchers must carry bottled oxygen.

The CBI hardware was designed primarily by Stephen Padin, the project’s chief scientist, and its software by senior research associate Timothy Pearson and staff scientist Martin Shepherd. Postdoctoral scholar Brian Mason and graduate students John Cartwright, Jonathan Sievers, and Patricia Udomprasert also played critical roles.

In five papers submitted to the Astrophysical Journal, the Caltech team, along with collaborators from CITA, the National Radio Astronomy Observatory, the University of Chicago, Universidad de Chile, the University of Alberta, UC Berkeley, and the Marshall Space Flight Center, reports on observations of the CMB obtained since the CBI began operation in January 2000.

The CBI is supported by Caltech, the National Science Foundation, and the Canadian Institute for Advanced Research, and has also received generous support from Maxine and Ronald Linde, Cecil and Sally Drinkward, Stanley and Barbara Rawn, Jr., and the Kavli Institute.

Images and other information are available at www.astro.caltech.edu/~tjp/CBI/press/press.html.
•